Enhanced chattering reduction in re-entry phase of RLV using disturbance observer–based fractional-order ISMC with fixed-time convergence

This research focuses on the challenges of controlling the attitude of reusable launch vehicle (RLV) during their re-entry phase under inherent nonlinear dynamics and adverse environmental circumstances. In this context, a fixed-time disturbance observer (DO) with fractional-order integral sliding mode control (ISMC) technique is proposed, ensuring that the convergence of the state remains unaffected by the initial condition. The DO helps in estimating all the system uncertainties within an upper-bound fixed time frame. It then feeds this estimated value directly into the composite controller for disturbance attenuation. On the other hand, the fractional-order ISMC affirms the convergence of both the sliding surface and the state trajectories within a fixed time frame. The ISMC can work with a significantly small switching gain by incorporating the DO, which results in alleviation of input chattering problem from the ISMC design. To prove the fixed-time convergence of the proposed scheme, separate Lyapunov analyses are conducted for DO and the overall system. The effectiveness of the proposed scheme is validated through numerical analysis and performance comparisons. This analysis uses various metrics to assess its strength, namely, transient and steady-state responses, accuracy, control effort, invariance against disturbances, and input chattering suppression.